Date:	Thu, 26 Aug 2010 21:28:37 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	estellnb@...il.com
Cc:	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: block network access for certain users/groups

Tetsuo Handa wrote:
> Elmar Stellnberger wrote:
> > I need to block network access for certain users/groups, fully:
Oh, I thought you want to partially block communication by specific user.

You want to entirely block communication by specific user? Then, it is easy.
You can block all socket syscalls by that user using LSM.
I made an example LSM module which can be added as a loadable kernel module
( http://www.youtube.com/watch?v=wG8BTLMu5wo ). You can modify it as you like.

You can compile and load below module provided that your kernel config has
CONFIG_SECURITY_NETWORK=y .

Did this module answer your question?

Thanks.
----------------------------------------
/*
 * Uid-NoSock Linux Security Module
 *
 * Author: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
 *
 * This module blocks socket syscalls by specific UID. Use at your own risk.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2, as
 * published by the Free Software Foundation.
 */
/*
 * Place this file (uid_nosock.c) in some directory and create Makefile
 * containing a line
 *
 *  obj-m += uid_nosock.o
 *
 * and run
 *
 *   make SUBDIRS=directory_containing_this_file/ modules modules_install
 *
 */
#include <linux/security.h>
#ifndef CONFIG_SECURITY_NETWORK
#error This module depends on CONFIG_SECURITY_NETWORK=y
#endif
#ifndef CONFIG_SECURITY
#error This module depends on CONFIG_SECURITY=y
#endif
#ifndef CONFIG_KALLSYMS
#error This module depends on CONFIG_KALLSYMS=y
#endif
#ifndef CONFIG_MODULES
#error This module depends on CONFIG_MODULES=y
#endif
#include <linux/module.h>
#include <linux/pid_namespace.h>
#include <linux/mount.h>
#include <linux/fs_struct.h>
static struct security_operations original_security_ops;

/* UID to block socket syscalls. */
#define UID_NOSOCK_REJECT_UID 500

static bool uid_nosock_deny(void)
{
	return current_uid() == UID_NOSOCK_REJECT_UID;
}

static int uid_nosock_socket_create(int family, int type, int protocol,
				    int kern)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_create(family, type, protocol,
						   kern);
}

static int uid_nosock_socket_bind(struct socket *sock,
				  struct sockaddr *address, int addrlen)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_bind(sock, address, addrlen);
}

static int uid_nosock_socket_connect(struct socket *sock,
				     struct sockaddr *address, int addrlen)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_connect(sock, address, addrlen);
}

static int uid_nosock_socket_listen(struct socket *sock, int backlog)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_listen(sock, backlog);
}

static int uid_nosock_socket_accept(struct socket *sock,
				    struct socket *newsock)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_accept(sock, newsock);
}

static int uid_nosock_socket_sendmsg(struct socket *sock, struct msghdr *msg,
				     int size)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_sendmsg(sock, msg, size);
}

static int uid_nosock_socket_recvmsg(struct socket *sock, struct msghdr *msg,
				     int size, int flags)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_recvmsg(sock, msg, size, flags);
}

static int uid_nosock_socket_getsockname(struct socket *sock)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_getsockname(sock);
}

static int uid_nosock_socket_getpeername(struct socket *sock)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_getpeername(sock);
}

static int uid_nosock_socket_getsockopt(struct socket *sock, int level,
					int optname)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_getsockopt(sock, level, optname);
}

static int uid_nosock_socket_setsockopt(struct socket *sock, int level,
					int optname)
{
	if (uid_nosock_deny())
		return -EPERM;
	return original_security_ops.socket_setsockopt(sock, level, optname);
}

/* Find symbols from /proc/kallsyms . */
static void *__init uid_nosock_find_symbol(const char *keyline)
{
	struct file *file;
	char *buf;
	unsigned long entry = 0;
	{
		struct fs_struct *fs = current->fs;
		struct vfsmount *mnt = current->nsproxy->pid_ns->proc_mnt;
		struct path old_path;
		struct path new_path = { mnt, mnt->mnt_root };
		spin_lock(&fs->lock); /* was write_lock() till 2.6.35. */
		old_path = fs->pwd;
		fs->pwd = new_path;
		spin_unlock(&fs->lock);
		file = filp_open("kallsyms", O_RDONLY, 0);
		spin_lock(&fs->lock);
		fs->pwd = old_path;
		spin_unlock(&fs->lock);
	}
	if (IS_ERR(file))
		goto out;
	buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
	if (buf) {
		int len;
		int offset = 0;
		while ((len = kernel_read(file, offset, buf,
					  PAGE_SIZE - 1)) > 0) {
			char *cp;
			buf[len] = '\0';
			cp = strrchr(buf, '\n');
			if (!cp)
				break;
			*(cp + 1) = '\0';
			offset += strlen(buf);
			cp = strstr(buf, keyline);
			if (!cp)
				continue;
			*cp = '\0';
			while (cp > buf && *(cp - 1) != '\n')
				cp--;
			entry = simple_strtoul(cp, NULL, 16);
			break;
		}
		kfree(buf);
	}
	filp_close(file, NULL);
out:
	return (void *) entry;
}

/* Never mark this variable as __initdata . */
static struct security_operations *uid_nosock_security_ops;

/* Never mark this function as __init . */
static int uid_nosock_addr_calculator(struct file *file)
{
	return uid_nosock_security_ops->file_alloc_security(file);
}

static int __init uid_nosock_init(void)
{
	struct security_operations **ptr;
	struct security_operations *ops;
	int i;
	const u8 *cp;
	/*
	 * Guess "struct security_operations *security_ops;".
	 * This trick depends on below assumptions.
	 *
	 * (1) Compiler generates identical code for security_file_alloc() and
	 *     uid_nosock_addr_calculator().
	 * (2) uid_nosock_security_ops is found within 128 bytes from
	 *     uid_nosock_addr_calculator, even if additional code (e.g. debug
	 *     symbols) is added.
	 * (3) It is safe to read 128 bytes from uid_nosock_addr_calculator.
	 * (4) uid_nosock_security_ops != Byte code except
	 *     uid_nosock_security_ops.
	 */
	cp = (const u8 *) uid_nosock_addr_calculator;
	for (i = 0; i < 128; i++) {
		if (sizeof(uid_nosock_security_ops) == sizeof(u32)) {
			if (*(u32 *) cp == (u32) &uid_nosock_security_ops)
				break;
		} else if (sizeof(uid_nosock_security_ops) == sizeof(u64)) {
			if (*(u64 *) cp == (u64) &uid_nosock_security_ops)
				break;
		}
		cp++;
	}
	if (i == 128) {
		printk(KERN_ERR
		       "Can't resolve uid_nosock_security_ops structure.\n");
		return -EINVAL;
	}
	cp = (const u8 *) uid_nosock_find_symbol(" security_file_alloc\n");
	if (!cp) {
		printk(KERN_ERR "Can't resolve security_file_alloc().\n");
		return -EINVAL;
	}
	/* This should be "struct security_operations *security_ops;". */
	ptr = *(struct security_operations ***) (cp + i);
	if (!ptr) {
		printk(KERN_ERR "Can't resolve security_ops structure.\n");
		return -EINVAL;
	}
	ops = *ptr;
	if (!ops) {
		printk(KERN_ERR "No security_operations registered.\n");
		return -EINVAL;
	}
	memmove(&original_security_ops, ops, sizeof(original_security_ops));
	smp_mb();
	BUG_ON(ops->socket_create == uid_nosock_socket_create ||
	       ops->socket_bind == uid_nosock_socket_bind ||
	       ops->socket_connect == uid_nosock_socket_connect ||
	       ops->socket_listen == uid_nosock_socket_listen ||
	       ops->socket_accept == uid_nosock_socket_accept ||
	       ops->socket_sendmsg == uid_nosock_socket_sendmsg ||
	       ops->socket_recvmsg == uid_nosock_socket_recvmsg ||
	       ops->socket_getsockname == uid_nosock_socket_getsockname ||
	       ops->socket_getpeername == uid_nosock_socket_getpeername ||
	       ops->socket_getsockopt == uid_nosock_socket_getsockopt ||
	       ops->socket_setsockopt == uid_nosock_socket_setsockopt);
	ops->socket_create = uid_nosock_socket_create;
	ops->socket_bind = uid_nosock_socket_bind;
	ops->socket_connect = uid_nosock_socket_connect;
	ops->socket_listen = uid_nosock_socket_listen;
	ops->socket_accept = uid_nosock_socket_accept;
	ops->socket_sendmsg = uid_nosock_socket_sendmsg;
	ops->socket_recvmsg = uid_nosock_socket_recvmsg;
	ops->socket_getsockname = uid_nosock_socket_getsockname;
	ops->socket_getpeername = uid_nosock_socket_getpeername;
	ops->socket_getsockopt = uid_nosock_socket_getsockopt;
	ops->socket_setsockopt = uid_nosock_socket_setsockopt;
	printk(KERN_INFO "Uid-Nosock: Blocking socket syscalls by uid=%u.\n",
	       UID_NOSOCK_REJECT_UID);
	return 0;
}

module_init(uid_nosock_init);
MODULE_LICENSE("GPL");
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